This invention relates to a DC/AC converter, more particularly a bridge circuit for gas discharge lamps comprising an integrated circuit having connection terminals for the application of a high direct voltage and a low direct voltage, a first insulated gate field effect transistor, whose drain is connected to a first connection terminal, and a second insulated gate field effect transitor, whose source is connected to the second connection terminal, the source of the first field effect transistor and the drain of the second field effect transistor both being connected to an output terminal. A control circuit is provided by means of which an alternating control signal can be supplied to the gate of the second field effect transistor and in inverted form to the gate of the first field effect transistor, as a result of which the first and second field effect transistors in push-pull can be made conducting and non-conducting. This control circuit comprises an inverter stage having a third insulated gate field effect transistor, whose source is connected to the second connection terminal, whose gate is connected to the gate of the second field effect transistor and whose drain is connected to a load element and to the gate of the first field effect transistor.
Such a converter is known inter alia from the article "A Versatile 250/300-V IC Process for Analogue and Switching Applications" by A. W. Ludikhuize, published in I.E.E.E. Transactions on Electron Devices, Vol. ED-33, No. 12, December 1986, pp. 2008/2015.
As is known, the stability and the efficiency of gas discharge lamps marketed, for example, under the designations SL and PL can be considerably improved when they are operated with an alternating voltage whose frequency is considerably higher than that of the mains voltage, for example, a frequency between 1 and 100 kHz. The bridge circuit serves to convert a direct voltage, which can be obtained by means of a rectifier circuit from the supply mains, into this high-frequency alternating voltage. In principle, this bridge circuit is composed of two switches, which are connected in series between a high and a low supply voltage and are operated in push-pull and connect the output terminal alternately to the high and the low supply voltage. In the case in which only such a branch of switches is present between the positive and the negative supply, the other side of the discharge lamp is kept at a DC level halfway between the positive and the negative supply. This embodiment, the so-called half bridge circuit, is simplest and ensures that half of the supply voltage is available for the lamp. The other embodiment, as shown, for example, in FIG. 14 of the aforementioned publication comprises two branches of switches, the lower switch of one branch being switched on and off simultaneously with the upper switch of the other branch. This so-called full bridge, whose construction and operation are more complicated than the half bridge, ensures that the whole supply voltage is available for the lamp. It will be clear without further explanation from the following description that the invention can be used both in full bridge circuits and in half bridge circuits.
In the full bridge circuit described in the aforementioned publication, the switches are constituted by n-type channel MOS transistors of practically identical construction. The source of one transistor (T2/T4) and the drain of the other transistor (T1/T3) are connected to the negative (O V) and the positive supply, respectively. The other source and drain zones are both connected to the output terminal (1/02). The inverter transistor (T5/T6) is also constituted by an n-channel MOS transistor, whose source is connected to the negative supply (O V). When a positive voltage is applied to the gate or control electrode of the inverter transistor, this transistor becomes conducting and the control electrode of the said other transistor (T1/T3) is connected to the low supply (ground). This transistor is then non-conducting (closed). The positive control signal is then applied to the control electrode of the said one transistor (T2/T4), which is then in the conductive state. The output terminal is then at the low level. In the other situation, in which a low control signal is supplied to the control electrode of the inverter transistor and to the one transistor (T2/T4) so that these transistors are non-conducting, the control signal should not only be inverted, but also the voltage level has to be shifted in order to guarantee that the voltage between the control electrode and the source of the transistor (T1/T3) remains higher than the threshold voltage. This shift in level is obtained by means of a so-called bootstrap circuit between the output terminal (01/02) and the control electrode of the transistor (T1/T3) and by means of the rectifier diodes, as a result of which a control voltage can be obtained which lies almost 15 V above the supply voltage.
The bootstrap circuit comprises a 15 V auxiliary voltage, a diode, a capacitor and a resistor. This resistor of, for example, 5 k .OMEGA. has a double function. In the first place, it permits the application of a voltage across the bootstrap capacitor when the inverter transistor is conducting and hence a current flows through the inverter circuit. A second effect of this resistor is that together with the parasitic capacitance of the control electrode of T1/T3 an RC time is formed which, when T1/T3 is switched on, causes a delay, so that in each branch both switches can not simultaneously conduct. For the last-mentioned reason, the control electrode of T2/T4 is also connected to a resistor and a diode is connected across this resistor so that the process of switching off T2/T4 is effected rapidly.
Experiments with the bridge circuit described here have shown that the current consumption and the dissipation in the inverter circuit are fairly high when, for the sake of simplicity, the 15 V auxiliary voltage is derived via a resistor from the high direct voltage, as a result of which the temperature of the circuit can rise to an undesired high level. Because of the high cost involved, it is generally not possible to reduce the temperature by a better cooling system. A reduction of the dissipation by means of a higher load resistor--in the said bridge circuit 5k .OMEGA.-- is hardly possible because of the higher RC times introduced thereby, as a result of which the transistor connected to the positive supply would be switched on again too late.